RFID Systems Using Distributed Exciter Network

ABSTRACT

RFID systems are disclosed that include at least one RFID receiver system and a distributed exciter architecture. Exciters can be connected via wired and/or wireless connections to the RFID receiver system, which can control activation of the exciters to detect the presence of RFID tags within interrogation spaces defined by the exciter topology. One embodiment includes an RFID receiver system configured to detect information from RFID tags within a receive coverage area, and a plurality of exciters defining a plurality of interrogation spaces within the receive coverage area of the receiver system. The receiver system is configured to transmit a control signal that identifies one of the exciters and includes information indicative of an RFID tag interrogation signal, the exciters are configured to receive the control signal, and the exciter identified in the control signal is configured to illuminate an interrogation space with the RFID tag interrogation signal.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/633,623 filed Jun. 26, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/213,851 filed Mar. 14, 2014 and issued as U.S.Pat. No. 9,690,957 on Jun. 27, 2017, which is a continuation of U.S.patent application Ser. No. 13/757,688 filed Feb. 1, 2013 and issued asU.S. Pat. No. 8,680,970 on Mar. 25, 2014, which is a continuation ofU.S. patent application Ser. No. 12/054,331 filed Mar. 24, 2008 andissued as U.S. Pat. No. 8,395,482 on Mar. 12, 2013, which claimspriority to U.S. Provisional Patent Application No. 60/896,864 filedMar. 23, 2007, the disclosures of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates generally to RFID systems and more specificallyto an RFID system that incorporates at least one RFID receiver systemand a distributed exciter architecture that defines a plurality ofinterrogation spaces.

The detection of signals in difficult environments, such as where thesignal to noise ratio is very low and/or the interference from othersignals is very high, has always been a challenging problem.

In RFID systems such as the RFID systems described in U.S. patentapplication Ser. No. 11/971,678, entitled “RFID System with LowComplexity Implementation and Pallet Coding Error Correction,” filedJan. 9, 2008, the disclosure of which is incorporated by referenceherein in its entirety, RFID receiver subsystems rely on an enhanced RFfront end as well as processing capabilities, for detecting very lowpower signals in the presence of additive white Gaussian noise withfurther channel distortions in in-door or out-door wireless propagationchannels. These techniques are particularly applicable to RadioFrequency Identification (RFID) based systems. FIG. 12 illustrates atransmit and receive RFID reader similar to the transmit and receivereaders described in U.S. patent application Ser. No. 11/971,678 and thespecifics of the RF front end. The reader (12-9) follows an RFID tagprotocol to communicate with tags (12-5) using the same transmit andreceive frequencies. The time-line showing this communication is shownin FIG. 13. The protocol governs the reader transmission of (12-4) data(13-2) and a continuous waveform (CW) (13-4) to the tag, and reception(12-2) of the tag's data (13-10). From FIG. 13, during the period thatthe tag is backscattering a packet to the reader, the reader istransmitting CW signal (13-4). That is the received signal is acomposite of transmitted CW and received tag signal (12-7). The receiversubsystem (12-9) performs baseband down conversion (12-12, 12-14),filtering (12-16, 12-18), and amplification (12-20, 12-22). At theoutput of the baseband amplifiers the signal from the tag is present, aswell as a strong DC component. This DC component is canceled by using DCblock capacitors (12-27, 12-29). To further improve the performance ofthe DC cancellation, the input to the DC block capacitors (12-27, 12-29)is controlled through a switch (12-24, 12-26) which is only closedduring the period that the system is receiving data from the tag, asdepicted in FIG. 13 (13-10). The digital processor (12-46), whichmaintains the system timing control of the switch control, opens theswitch during the reader transmit periods (13-2, 13-6, 13-14, 13-18),and closes the switches during the expected receive periods (13-4,13-10,13-16). The output of the DC cancellation capacitors is followed by theAGC loops (12-32, 12-34), analog to digital converters (12-36, 12-38),and digital processor (12-40), which includes the control algorithms.

SUMMARY OF THE INVENTION

RFID systems in accordance with many embodiment of the invention includeone or more RFID receiver systems that are associated with a number ofdistributed transmitters, referred to as RFID tag exciters (or just“exciter”). The exciters can act as signal repeaters from the RFIDreceiver system that enable transmission of a tag signal to a distantexciter, which in turn filters, amplifies and re-transmits the signal tothe intended collection of RFD tags within the line-of-sight view of theexciter. The logical interconnect and communications topology scalesfrom a centralized point of control up to a fully connected graph.Physically, the communications network can be either wired lines orwireless.

Each exciter may or may not embed active re-generation of thetransmitted signal to the RFID tag; however, in many embodiments eachexciter emits sufficient power and a waveform compatible with therequirements of a standard such as set forth by Electronic Product CodeGlobal (EPC Global) or International Standard Organizations (ISO). Thetransmission from the RFID receiver system to an exciter may becompatible with these standards and/or utilize other waveformscompatible with regulatory requirements such as set forth by US FederalCommunication Commission (FCC) or other international regulatoryagencies.

One embodiment of the invention includes an RFID receiver systemconfigured to detect information from RFID tags within a receivecoverage area, and a plurality of exciters defining a plurality ofinterrogation spaces within the receive coverage area of the RFIDreceiver system. In addition, the RFID receiver system is configured totransmit a control signal that identifies one of the plurality ofexciters and includes information indicative of an RFID taginterrogation signal, the plurality of exciters are configured toreceive the control signal, and the exciter identified in the controlsignal is configured to illuminate an interrogation space with the RFIDtag interrogation signal.

In a further embodiment of the invention, the RFID receiver systemcommunicates to at least one of the plurality of exciters via a wiredconnection.

In another embodiment of the invention, the wired connection directlyconnects the exciter to the RFID receiver system.

In a still further embodiment of the invention, the wired connectionconnects the exciter to another of the plurality of exciters, which isconfigured to relay control signals from the RFID receiver system viathe wired connection.

In still another embodiment of the invention, the wired connection is acoaxial cable, and the control signal is modulated at a first RFfrequency.

In a yet further embodiment of the invention, the exciter is configuredto down convert the control signal to extract at least the identity ofthe exciter identified by the control signal, and the exciter isconfigured to up convert and transmit the RFID tag interrogation signalat a second RF frequency, when the exciter is the exciter identified bythe control signal.

In yet another embodiment of the invention, the control signal specifiesthe frequency of the second RF frequency.

In a further embodiment of the invention again, the second RF frequencyis the same as the first RF frequency.

In another embodiment of the invention again, the wired connection is atwisted pair cable, and the control signal is a baseband signal.

In a further additional embodiment of the invention, the exciter isconfigured to extract at least the identity of the exciter identified bythe control signal, and the exciter is configured to up convert andtransmit the RFID tag interrogation signal at a transmit RF frequency,when the exciter is the exciter identified by the control signal.

In another additional embodiment of the invention, the control signalspecifies the transmit RF frequency.

In a still yet further embodiment of the invention, the RFID receiversystem communicates with at least one of the plurality of exciters via awireless connection.

In still yet another embodiment of the invention, the wirelessconnection is a direct connection between the RFID receiver system andthe exciter.

In a still further embodiment of the invention again, the wirelessconnection is between the exciter and a second of the plurality ofexciters.

In still another embodiment of the invention again, the second of theplurality of exciters is configured to relay control signals from theRFID receiver system via the wireless connection.

In a still further additional embodiment of the invention, the controlsignal is modulated at a first RF frequency, and the exciter isconfigured to down convert the control signal to extract at least theidentity of the exciter identified by the control signal.

In still another additional embodiment of the invention, the exciter isconfigured to up convert and transmit the RFID tag interrogation signalat a second RF frequency when the exciter is the exciter identified bythe control signal.

In a yet further embodiment of the invention again, the control signalspecifies the frequency of the second RF frequency.

In yet another embodiment of the invention again, the exciter isconfigured to generate an RFID tag interrogation signal and modulate theRFID tag interrogation signal onto a second RF frequency.

In a yet further additional embodiment of the invention, the controlsignal specifies the frequency of the second RF frequency.

In yet another additional embodiment of the invention, the RFID receiversystem is configured to transmit a status signal at a first RF frequencythat identifies one of the plurality of exciters, the exciter isconfigured to extract at least the identity of the exciter identified bythe status signal, and the exciter is configured to generate a responsesignal and transmits the response signal at a second RF frequency.

In a further additional embodiment of the invention again, the waveformof the response signal is similar to the waveform generated by anilluminated RFID tag.

In another additional embodiment of the invention again, the statussignal specifies the frequency of the second RF frequency.

In another further embodiment of the invention, the RFID receiver systemis configured to transmit control signals that activate a plurality ofthe exciters to transmit RFID tag interrogation signals at differentfrequencies.

In still another further embodiment of the invention, the controlsignals transmitted by the RFID receiver system cause the plurality ofactivated exciters to transmit in accordance with a frequency hoppingprotocol.

In yet another further embodiment of the invention, the RFID receiversystem is configured to allocate frequencies to exciters randomly.

In another further additional embodiment of the invention, the RFIDreceiver system possesses information concerning the exciterdistribution topology, and the RFID receiver system uses the topologyinformation when allocating frequencies to activated exciters.

In another further embodiment of the invention again, the control signalincludes an n-bit address.

In still yet another further embodiment of the invention, the controlsignal includes all the necessary signal characteristics and parametersto generate the desired waveform output from the exciter.

In still another further additional embodiment of the invention, thecontrol signal includes information that can be used by an exciter toperform transmit power calibration.

In still another further embodiment of the invention again, the controlsignal includes information indicative of a transmission frequencyselection.

Yet another further additional embodiment of the invention also includesa second RFID receiver system configured to detect information from RFIDtags within a second receive coverage area, and a plurality of excitersdefining a plurality of interrogation spaces within the receive coveragearea of the second RFID receiver system. In addition, the second RFIDreceiver system is configured to transmit a control signal thatidentifies one of the plurality of exciters and includes informationindicative of an RFID tag interrogation signal, the plurality ofexciters within the coverage area of the second RFID receiver system areconfigured to receive the control signal from the second RFID receiversystem, and the exciter identified in the control signal is configuredto illuminate an interrogation space within the coverage area of thesecond RFID receiver system with the RFID tag interrogation signal.

In yet another further embodiment of the invention again, the RFIDreceiver system is configured to detect RFID tag information when anexciter illuminates an interrogation space with an RFID taginterrogation signal, and the RFID receiver system is configured todetermine whether the detected information is from an RFID tag locatedwithin the interrogation space illuminated by the exciter.

Another further additional embodiment of the invention again includes asensor located within the illuminated interrogation space configured todetect changes within the interrogation space. In addition, the sensoris configured to communicate sensor output to the RFID receiver system,and the RFID receiver system is configured to determine whether thedetected information is from an RFID tag located within theinterrogation space illuminated by the exciter using informationincluding the sensor output.

In still yet another further additional embodiment of the invention, theRFID receiver system is configured to detect RFID tag information whenother exciters illuminate other interrogation spaces with RFID taginterrogation signals, and the RFID receiver system is configured todetermine whether the detected information is from an RFID tag locatedwithin the interrogation space illuminated by the exciter usinginformation including the RFID tag information detected when otherexciters illuminated other interrogation spaces.

In still yet another further embodiment of the invention again, the RFIDtag information is an RF signal, the RFID receiver system is configuredto collect information concerning features of the RFID tag informationRF signal, and the RFID receiver system is configured to determinewhether the detected information is from an RFID tag located within theinterrogation space illuminated by the exciter using informationincluding the features of the RFID tag information RF signal.

In yet another further additional embodiment of the invention again, thecollected features of the RFID tag information RF signal include signalstrength, signal-to-noise ratio, and direction of arrival.

In a still yet further additional embodiment of the invention, the RFIDreceiver system repeatedly causes a plurality of exciters tosequentially illuminate a plurality of interrogation spaces and recordsthe detection of RFID tag information, and the RFID receiver system isconfigured to determine whether detected information is from an RFID taglocated within an interrogation space illuminated by one of theplurality of exciters using information including the rate at which theRFID tag information is detected when the interrogation space isilluminated.

In still yet another additional embodiment of the invention, the RFIDreceiver system possesses information concerning the exciter topography,the RFID receiver system is configured to estimate expected detectionrates for RFID tags in different interrogation spaces, and the RFIDreceiver system is configured to determine whether detected informationis from an RFID tag located within an interrogation space illuminated byone of the plurality of exciters using information including the rate atwhich the RFID tag information is detected when the interrogation spaceis illuminated and the expected detection rates for RFID tags indifferent interrogation spaces.

In a yet further additional embodiment of the invention again, the RFIDreceiver system is configured to determine movement of an RFID tag fromone interrogation space to another interrogation space using informationincluding the rate at which the information is detected wheninterrogation spaces are illuminated and the expected detection ratesfor RFID tags in different interrogation spaces.

An exciter configured to illuminate an interrogation space in accordancewith an embodiment of the invention includes, an input configured toreceive a control signal including an exciter address and informationindicative of an RFID tag interrogation signal, and a transmitterconfigured to transmit an RFID tag interrogation signal, a decode moduleconfigured to control the transmitter to transmit the RFID taginterrogation signal indicated by a control signal, when the exciter isaddressed by the exciter address in the control signal.

In a further embodiment of the invention, the input includes a coaxialcable connector, the control signal is modulated on a first RFfrequency, and the transmitter is configured to modulate the RFID taginterrogation signal onto a second RF frequency.

In another embodiment of the invention, the frequency of the second RFfrequency is specified by the control signal.

In a still further embodiment of the invention, the frequency of thesecond RF frequency is the same as the frequency of the first RFfrequency.

Still another embodiment of the invention also includes an output, wherethe output includes a coaxial cable connector, and a coupler connectedto the input, the decode module, and the output, where the coupler isconfigured to split the input signal between the decode module and theoutput.

In a yet further embodiment of the invention, the input includes atwisted pair connector, the control signal is a baseband signal, and thetransmitter is configured to modulate the RFID tag interrogation signalonto an RF frequency.

In yet another embodiment of the invention, the frequency of the RFfrequency is specified by the control signal.

A further additional embodiment of the invention includes an output,where the output includes a twisted pair connector, and a couplerconnected to the input, the decode module, and the output, where thecoupler is configured to split the input signal between the decodemodule and the output.

In another additional embodiment of the invention, the input isconnected to a receive antenna, the control signal is a wireless signaltransmitted at a first RF frequency, and the transmitter is configuredto modulate the RFID tag interrogation signal onto a second RFfrequency.

In a further embodiment of the invention again, the frequency of thesecond RF frequency is specified by the control signal.

In another embodiment of the invention again, the decode module isconfigured to control the transmitter to retransmit the control signal,when the exciter is not addressed by the exciter address in the controlsignal.

In a still yet further embodiment of the invention, the transmitterincludes a power amplifier, and a level control loop configured tomonitor the output of the power amplifier to adjust the gain of thepower amplifier to maintain the output signal below a predeterminedthreshold.

In still yet another embodiment of the invention the transmitterincludes a power amplifier, and a level control loop configured todetect the power of input signals to the power amplifier and the outputof the power amplifier and to adjust the gain of the power amplifier tomaintain the output signal below a predetermined threshold.

An embodiment of the method of the invention includes generating acontrol signal including an exciter address and an RFID taginterrogation signal, transmitting the control signal to at least one ofthe plurality of distributed exciters, illuminating an interrogationspace with the RFID tag interrogation signal using the exciter addressedby the control signal, and receiving an RFID tag information signal.

In a further embodiment of the method of the invention, generating acontrol signal includes identifying an n-bit address associated with anexciter located within an interrogation space.

In another embodiment of the method of the invention, generating acontrol signal includes determining signal characteristics andparameters of an RFID tag interrogation signal to be generated by anexciter.

In a still further embodiment of the method of the invention, generatinga control signal includes generating information that can be used by anexciter to perform transmit power calibration.

In still another embodiment of the method of the invention, generating acontrol signal includes generating information indicative of atransmission frequency selection.

In a yet further embodiment of the method of the invention, transmittingthe control signal includes transmitting the control signal to a firstexciter that relays the signal to a second exciter.

In yet another embodiment of the method of the invention, illuminatingthe interrogation space includes extracting the RFID tag interrogationsignal from the control signal, modulating the RFID tag interrogationsignal onto a transmission frequency, and transmitting the modulatedRFID tag interrogation signal.

In a further additional embodiment of the method of the invention,extracting the RFID tag interrogation signal further comprises downconverting the control signal.

In another additional embodiment of the method of the invention,modulating the RFID tag interrogation signal onto a transmissionfrequency further comprises modulating the RFID tag interrogation signalonto a transmission frequency specified by the control signal.

In a further embodiment again of the method of the invention, receivingan RFID tag information signal includes determining whether receivedRFID tag information is from an RFID tag located within the illuminatedinterrogation space.

In another embodiment again of the method of the invention, determiningwhether received RFID tag information is from an RFID tag located withinthe illuminated interrogation space includes detecting changes in theinterrogation space using at least one sensor.

In a still yet further embodiment of the method of the invention,determining whether received RFID tag information is from an RFID taglocated within the illuminated interrogation space includes detectingRFID tag information when other interrogation spaces are illuminated.

In still yet another embodiment of the method of the invention,determining whether received RFID tag information is from an RFID taglocated within the illuminated interrogation space includes collectinginformation concerning features of the RFID tag information signal.

In a still further additional embodiment of the method of the invention,the collected information concerning features of the RFID taginformation includes signal strength, signal-to-noise ratio, anddirection of arrival.

In still another additional embodiment of the method of the invention,determining whether received RFID tag information is from an RFID taglocated within the illuminated interrogation space includes repeatedlyilluminating a plurality of interrogation spaces, recording thedetection of RFID tag information, and determining the rate at whichRFID tag information is detected when each interrogation space isilluminated.

In a still further embodiment again of the method of the invention,determining whether received RFID tag information is from an RFID taglocated within the illuminated interrogation space includes estimatingexpected detection rates for RFID tags in different interrogation spacesusing information concerning the exciter topography, and comparing therate at which RFID tag information is detected when an interrogationspace is illuminated with expected detection rates for the interrogationspace when an RFID tag is located within different interrogation spaces.

A method of estimating the location of an RFID tag within a plurality ofinterrogation spaces in accordance with an embodiment of the method ofthe invention includes repeatedly illuminating each of the plurality ofinterrogation spaces, recording the illuminated interrogation space whenthe RFID tag is detected, and determining the rate at which the RFID tagis detected when each interrogation space is illuminated, and recordingthe rate at which the RFID tag is detected for each interrogation spaceand for each exciter.

A yet further additional embodiment of the method of the inventionincludes obtaining information concerning the topology of the pluralityof interrogation spaces, estimating expected detection rates for RFIDtags in different interrogation spaces using information concerning theinterrogation space topography, and comparing the rate at which RFID taginformation is detected when an interrogation space is illuminated withexpected detection rates for the interrogation space when an RFID tag islocated within different interrogation spaces.

In yet another additional embodiment of the method of the invention,comparing the observed detection rate to the estimated detection ratecomprises using the difference between the expected detection rate andobserved detection rate as the argument to a Gaussian density functionto produce a probability of the observed detection rate in a giveninterrogation space.

In a yet further embodiment again of the method of the invention,recording the rate at which RFID tag information is detected includesusing an exciter ID, a hypothesis region, and an RFID tag ID.

Yet another embodiment again of the method of the invention furtherincludes detecting movement of an RFID tag from a first interrogationspace to a second interrogation space using the comparison of the rateat which RFID tag information is detected and the expected detectionrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a network diagram of an RFID system including a distributedexciter architecture, where the exciters are connected to the RFIDsystem via cables, in accordance with an embodiment of the invention.

FIG. 2 is a network diagram of an RFID system including two RFIDreceiver systems and a distributed exciter architecture, where theexciters are connected to the RFID receiver system via cables, inaccordance with an embodiment of the invention.

FIG. 3 is a network diagram of an RFID system including two RFIDreceiver systems and a distributed exciter architecture, where theexciters communicate with the RFID receiver system wirelessly, inaccordance with an embodiment of the invention.

FIG. 4 is a semi-schematic circuit diagram of an exciter configured tobe connected to an RFID receiver system via a coaxial cable inaccordance with an embodiment of the invention.

FIG. 5 is a semi-schematic circuit diagram of an exciter configured tobe connected to an RFID receiver system via a twisted pair in accordancewith an embodiment of the invention.

FIG. 6 is a semi-schematic circuit diagram of an exciter configured towirelessly communicate with an RFID receiver system in accordance withan embodiment of the invention.

FIG. 7 is a semi-schematic circuit diagram of a wireless re-generativeexciter in accordance with an embodiment of the invention.

FIG. 8 is a plan view of an antenna element in accordance with anembodiment of the invention.

FIG. 9 is a cross-sectional view of an antenna assembly in accordancewith an embodiment of the invention.

FIG. 10 is a plan view of an array of receiver antennas in accordancewith an embodiment of the invention.

FIG. 11 is a cross-sectional view of an array of receiver antennas inaccordance with an embodiment of the invention that is similar to thearray shown in FIG. 10.

FIG. 12 is a semi-schematic circuit diagram of a transmit and receiveRFID reader.

FIG. 13 is a chart conceptually illustrating tag protocol timing.

FIG. 14 is a conceptual illustration of software used to configure anRFID application server in accordance with an embodiment of theinvention.

FIG. 15 is a flow chart of a process for determining whether RFID taginformation was detected from a tag within a predetermined interrogationspace in accordance with an embodiment of the invention.

FIG. 16 is a semi-schematic diagram showing an RFID system including adistributed exciter architecture in which exciters are paired tofacilitate tag read location discrimination in accordance with anembodiment of the invention.

FIGS. 17a and 17b are conceptual diagrams illustrating frequencyassignment and scheduling for a plurality of exciters in accordance withan embodiment of the invention.

FIG. 18 is a block diagram illustrating input level conditionmeasurement and an output level feedback control loop in accordance withan embodiment of the invention.

FIG. 19 is a block diagram showing a wireless or wireline controlledexciter with wireless status information backhaul enabling the wirelesscommunication of messages from an exciter to an RFID receiver system inaccordance with an embodiment of the invention.

FIG. 20 is a conceptual illustration of the movement of an RFID tagbetween two hypothesis regions.

FIG. 21 is a conceptual illustration of the spatial relationship betweenan exciter and an RFID tag that can be used to determine excitation linkmargin in an exciter/hypothesis topology, exciter transmit power,exciter radiation pattern, and typical RFID tag radiation pattern inaccordance with embodiments of the invention.

FIG. 22 is a chart illustrating a probability mass function thatdescribes the probability of RFID tag read rates for RFID tags locatedwithin a given hypothesis region under excitation by a given exciter inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Turning now to the drawings, RFID systems including at least one RFIDreceiver system and a distributed exciter architecture are shown. Inseveral embodiments, the desired overall interrogation space isdecomposed into a set of interrogation spaces and exciters are placed ineach target interrogation space. The RFID system obtains informationfrom collections of RFID tags in specific interrogation spaces bycontrolling the activation of exciters. An RFID tag within aninterrogation space is manipulated by illuminating the interrogationspace using an RFID tag interrogation signal provided to the exciter bythe RFID receiver system. The illuminated RFID tag backscattersinformation, which can be detected by the RFID receiver system.

The RFID receiver system can control the size of each interrogationspace by adjusting the total emitter power from the exciter. In a numberof embodiments, the overall performance of the system is improved byselecting each exciter transmit antenna type to provide the desiredlevel of directivity, thereby controlling the beam-width for the targetinterrogation space. In several embodiments, the exciters are connectedvia cables to the RFID receiver system. In a number of embodiments, theexciters are wirelessly connected to the RFID receiver system.

When an interrogation space topology has been defined, RFID systems inaccordance with embodiments of the invention can control theillumination of individual interrogation spaces to obtain locationinformation concerning items bearing RFID tags. In many embodiments, theRFID system polls exciters. In several embodiment, the RFID system canincorporate additional sensors that detect changes within aninterrogation space (e.g. movement) and the RFID system can activate thecorresponding exciter and/or exciters associated with adjacentinterrogation spaces to obtain information concerning any items bearingRFID tags moving between interrogation spaces.

A problem that can be encountered when using a distributed exciterarchitecture is the potential for an RFID tag to be read from outside ofan exciter's interrogation space (a false read). In several embodiments,information concerning various characteristics of the RFID system areused to detect the occurrence of false reads. In a number ofembodiments, data from sensors in the interrogation spaces, RFID taginformation detected in other interrogation spaces and/or the RFfeatures of the detected RFID tag information can be used to determinewhether a false read has occurred. In many embodiments, statisticalanalysis is used to detect false reads based upon predicted read ratesfor RFID tags located within the interrogation space. In theseembodiments, repeated illumination of an interrogation space and RFIDtag detection rates are compared to predicted detection rates todetermine the likely location of the RFID tag. In several embodiments,predicted detection rates are obtained using knowledge of an RFIDsystem's exciter topology.

An RFID system including a distributed exciter architecture inaccordance with an embodiment of the invention is shown in FIG. 1. TheRFID system (1-1) includes an RFID receiver system (1-2) connected to anarray of receiver antennas (1-4) and a plurality of exciters (1-6, 1-14,1-18, 1-23, 1-28) that are daisy chained to the RFID receiver system viacables (1-10, 1-9, 1-16, 1-22, 1-26). The RFID receiver system (1-2) isalso connected to a LAN (1-32) via connection (1-34). An RFIDapplication server (1-30) is connected to the LAN via connection (1-36).Although the plurality of exciters are shown as wired, in manyembodiments exciters communicate wirelessly with the RFID receiversystem.

In operation, the RFID receiver system (1-2) controls the activation ofexciters. The cable segments (1-10, 1-12, 1-16, 1-22, 1-26) carry bothdirect current (DC) power and control commands from the RFID receiversystem (1-2) to each exciter. The transmitted “backhaul signal” from theRFID receiver system (1-2) to the exciters embeds all the necessarysignal characteristics and parameters to generate a desired waveformoutput from the exciter module to an RFID tag. In several embodiments,each exciter can be commanded and addressed by an N-bit address,N-ranging from 16-to-32 bit. The exciters (1-8, 1-14, 1-18, 1-23, 1-28)can be operated sequentially or concurrently, depending on the number ofpossible beams the RFID receiver system can support. In the illustratedembodiment, the RFID receiver system (1-2) includes a single antennaarray (1-4) and is capable of generating a single beam. In otherembodiments, the RFID receiver system includes multiple antenna arraysand is capable of generating multiple beams (see discussion below).

The interrogation space and transmitted power of each exciter can bemanaged and controlled by the RFID receiver system (1-2). In theillustrated embodiment, the RFID receiver system (1-2) controls theexciters to create interrogation space (1-8, 1-15,1-20,1-24, & 1-28) ofdifferent sizes. In addition, the received coverage area isconfigurable. The RFID receiver system can receive signals from thecomplete coverage area (1-11). Alternatively, the RFID receiver systemcan adaptively beam-form to the specified exciter interrogation spaces(1-12, 1-21).

The RFID application server (1-30) schedules each exciter to operateharmoniously in multiple dimensions, which are time, frequency andspace. In a number of embodiments, the RFID application server (1-30)includes a scheduler for S/T/FDM (Space, Time and Frequency DivisionMultiplexing), which utilizes an optimization algorithm to maximize theprobability of successful manipulation of all the RFID tags within atarget interrogation space. In addition, the controller may utilizefrequency hopping in scheduling the frequency channel for each exciterin order to satisfy various regulatory constraints.

An exciter layout and a time line showing frequency channels assigned byan RFID application server in accordance with an embodiment of theinvention are shown in FIGS. 17a and 17b . The timeline (17-4), depictschannel selection within the 900 MHz ISM band (17-10), versus the timeline. In the illustrated embodiment at time ‘1’, exciters 12, 15, 7, 11,10, 3, 9, 4, 2, 1, 6, 14, 16, 8, 13, and 5 are activated for operationon channels 7, 16, 23, 25, 34, 40, 44, 46, 49, 50, 51, and 52respectively. Note that more than one exciter might occupy a givenchannel and to the extent that these exciters are topologicallyco-located (for instance exciters 7 and 11) collisions can occur.Therefore, better choices could have been made in the frequency/excitermapping plan. However, in the illustrated embodiment, random frequencieswere assigned to the active exciters (17-8), which implies thatco-location frequency collisions are possible (for instance on channel23 between exciters 7 and 11 at time instance 1). An algorithm that canoptimize frequency reuse and exciter location planning (14-12) (S/T/FDM)can be utilized to reduce overall interference degradation whileadhering to regulatory constraints. An RFID system including two RFIDreceiver systems and a distributed exciter architecture in accordancewith an embodiment of the invention is illustrated in FIG. 2. The RFIDsystem includes two RFID receiver systems (2-2, 2-20), each with aseparate array of receiver antennas (2-4, 2-22), and that are connectedto a LAN (2-21) via connections (2-23, 2-25). A plurality of exciters(2-6, 2-52, 2-58, 2-64, 2-32, 2-28, 2-38, 2-48, 2-44, 2-72, 2-70) areconnected to the two RFID receiver systems to create multipleinterrogation spaces (2-13, 2-54, 2-69, 2-66, 2-34, 2-28, 2-40, 2-46,2-49, 2-72,). In the illustrated embodiment, the exciters form twogroups, where each group is connected to the RFID receiver systems by aseparate cable (2-24, 2-30, 2-36, 2-42, 2-50 and 2-10, 2-12, 2-56, 2-62,2-68). The RFID application server (2-51) interfaces with the RFIDreceiver systems (2-2, 2-20), through the LAN (2-21), and manages theoperation of the exciters, which includes control, command,coordination, calibration and optimization of the exciter interrogationspaces. The functions of RFID application servers in accordance withembodiments of the invention are discussed further below.

As can be seen from the embodiment illustrated in FIG. 2, each RFIDreceiver system has a separate receive coverage area (2-5, 2-29).Therefore, the use of multiple RFID receiver systems can enable anincrease in the coverage area of the system. In addition, exciters indifferent coverage areas can be activated simultaneously. In theillustrated embodiment, several exciters (2-32, 2-44) occupy locationswithin the coverage area of both of the RFID receiver systems. Beamforming at each RFID receiver system allows a null to be placed in theother RFID receiver system's interrogation space hence avoidingcollisions when two exciters (and RFID tags) transmit simultaneously inthe different interrogation spaces.

An RFID system including multiple RFID receiver systems and adistributed exciter architecture constructed using exciters thatwirelessly communicate with the RFID receiver systems in accordance withan embodiment of the invention is illustrated in FIG. 3. The RFID systemis similar to the system shown in FIG. 2 with the exception that theexciters are configured to communicate wirelessly with the RFID receiversystems. Each RFID receiver system (2-2′, 2-20′) has both receive (2-4′,2-22′) and transmit antennas (3-6, 3-8). The transmit antenna radiatesthe forward link to the exciter, while the receive antenna arrayreceives the signal from RFID tags, within the interrogation space ofeach exciter. The forward link also carries exciter identification (ID)number, command, control and management information. In the illustratedembodiment, the receive coverage areas for the two RFID receiver systems(2-5′, 2-29′) is shown. The overlap region between the coverage areas(2-5′, 2-29′) is managed through receive array beam-forming andfrequency or time coordination of exciter operation. The RFIDapplication server (3-7) interfaces with the two RFID receiver systems(2-2′, 2-20′) through the LAN (2-21′) and manages the operations of theexciters that includes control, command, coordination, and calibrationof the exciters as well as optimization of the interrogation spaces.

Implementations of wired exciters in accordance with embodiments of theinvention are shown in FIGS. 4 and 5. FIG. 4 depicts an exciter inaccordance with an embodiment of the invention that is configured toconnect to an RFID system via coaxial cable. FIG. 5 depicts an exciterin accordance with an embodiment of the invention that is configured toconnect to an RFID system via universal twisted pair (UTP) in which theexciter can be connected via CAT-5 or 6 wires.

In the embodiment illustrated in FIG. 4, a reader coaxial interface(4-2, 4-4, 4-6, 4-8, 4-12) and an interface (4-14) for daisy chaining aplurality of exciters (4-52) are shown. A signal received from an RFIDreceiver system typically carries control signals, which are processedby the demodulate and decode command and control messages module (4-10)and used to control the self calibration and automatic level control(ALC) loop (4-18, 4-20, 4-48, 4-34, 4-36, 4-32) or to set the transmitpower in response to a manual setting (4-38, 4-30). Command and controlmessages for a wired exciter include messages that cause the exciter togo through power calibration (described below), to turn the transmitsignal on or off, and to control the reporting of exciter statusinformation (see description of hybrid wireless/wired exciter providedbelow with respect to FIG. 19). The received RF signal is split using acoupler (4-8) between the next exciter (4-52) and the transmit RF chain(4-40). The RF chain includes the receive low noise amplifier (4-16) andthe ALC loop (4-22,4-20,4-24, 4-48) consisting of the feedback loop andthe power amplifier (PA) (4-24). Output of the PA is followed by thetransmit antenna patch (4-28). The exciter additionally has interfacesto one or more external alarms (4-58), which are interfaced to theinternal processor (4-47).

A self calibration and automatic level control loop in accordance withan embodiment of the invention is shown in FIG. 18. This block measuresboth input (18-1) and output (18-4) power levels. Input power levelsbetween −10 dB Milliwatts and 27 dB Milliwatts are deemed to be within atolerable range. If the input signal is within this range then theOutput Power Level Measurement (18-3) block feeds back a signal to anamplifier module (18-2) to control the power amplifier's gain such thata total output power level of 30 dB Milliwatts is achieved. This outputlevel is compliant with the FCC total radiated power restriction for afrequency hopped system operating in the 900 MHz band. In otherembodiments, the total output power level of the exciter can becontrolled in accordance with other application constraints. Theembodiment illustrated in FIG. 18 is particularly useful forapplications in which the exciter regenerates a received control signal.Wireless exciters that synthesize the RFID tag interrogation waveform inaccordance with several embodiments of the invention can utilize anautomatic control loop that simply monitors the output level and adjuststhe transmitter power accordingly.

Turning now to FIG. 5, an exciter having a UTP interface, using CAT-5 or6 cables (5-4, 5-72) is illustrated. When a UTP interface is used, theinterface between the exciter and the RFID system (5-4) is at baseband.The exciter detects the baseband signal and modulates it to thespecified RF frequency. When a coaxial interface is used, the interfaceis at RF, where the RF signal is repeated, and baseband carries thecontrol signals. The raw data, which the RFID receiver system transmitsto the exciter includes the control and command signals as well as thedata that is modulated and transmitted to the RFID tag. Similar to thecoaxial version, UTP exciters can be daisy chained (5-2, 5-4, 5-6, 5-8,5-10, 5-12, 5-14, 5-72, 5-74) using UTP connectors. The demodulate anddecode command and control messages module (5-18) decodes the commandand control data used for configuring the RF synthesizer (5-22), themodulator (5-26), the self calibration and ALC control loop (5-24), andtransmit power manual setting subsystem (5-20). The data decoder andmodulator (5-26) detects and re-modulates transmit data in accordancewith the relevant standard, and is followed by up conversion (5-28,5-60, 5-58) and automatic gain control (5-32, 5-34, 5-36, 5-24). Theamplified signal is provided via a connection (5-66) to the transmitantenna patch (5-40). The exciter additionally has interfaces (5-76) toone or more external alarms (5-80), which are interfaced to the internalprocessor (5-16).

A wireless exciter in accordance with an embodiment of the invention isillustrated in FIG. 6. An RFID receiver system communicates with thewireless exciter on one frequency using the transmit antenna (6-4),while receiving data from the tags on a different frequency using itsarray of receiver antennas. In the illustrated embodiment, the wirelessexciter acts as an RF repeater utilizing the 900 MHz ISM band. In otherembodiments, other frequency bands can be used. The RF signal is downconverted from the signal transmitted by the RFID receiver systemtransmit antenna (6-4), to baseband on the down conversion outputconnection (6-20), and then the baseband signal is up converted to aselect frequency on the up conversion output connection (6-30). Thespecified frequencies for down and up conversions are communicated tothe exciter using the RF command channel, using the 900 MHz ISM band byway of example. The same methodology can be employed in other frequencybands. The exciter patch antenna (6-6) is discussed further below andincludes two feeds for receiving and transmitting RF signals. The RFIDreceiver system transmits to the exciters command, control, and transmitfrequency plan information. The commands are detected by the demodulateand decode command and control messages module (6-74) and processed tocontrol the dual synthesizer (6-66), the self-calibration and ALC loop(6-54, 6-36, 6-50, 6-38), and the transmit power manual setting loop(6-56, 6-48).

Command and control messages for a wireless exciter can include messagesthat cause the exciter to go through power calibration, to turn thetransmit signal on or off, to control the reporting of exciter statusinformation (see discussion of hybrid wireless/wired exciter below), toselect a transmit frequency, and to set other parameters that definevarious transmit waveform characteristics. In other embodiments, thecommand and control messages can provide other instructions to anexciter.

The exciter configures the dual synthesizer (6-66) with the receive andtransmit frequencies in response to instructions received from the RFIDreceiver system. The received RF signal is down converted (6-18) andthen up converted (6-28) to the specified transmit frequency. Thetransmit power is set and calibrated using the self calibration and ALCmodule (6-54) through the control loops (6-52, 6-62, 6-50), and TX powercalibration setting sub-system (6-56) through the control loops (6-48,6-58). The RF path includes the required filters (6-22, 6-32, 6-42) andamplifiers (6-10,6-24,6-38) for maintaining signal integrity andquality. The output of the last stage (6-42) is followed by the antennaelement (6-46), connected to the exciters transmit feed (6-46). Theexciter additionally has interfaces to one or more external alarms(6-80), which are interfaced to the internal processor (6-70).

A re-generative wireless exciter configured to demodulate, and detectthe data provided by an RFID receiver system at a first frequency, andthen modulate and transmit the RF signal at a different frequency inaccordance with an embodiment of the invention is illustrated in FIG. 7.The forward link from the RFID receiver system to the exciter (7-2, 7-5)can carry control and command signals, as well as raw data. The readerforward link (7-5) to the exciter can use any modulation format such asspread spectrum or any simple suppressed carrier modulation. In theillustrated embodiment, the operating frequency and transmitted power ofthis link are configured to meet regulatory requirements. For example,U.S. Federal Communication Commission (FCC) standards specified in FCCPart-15 can be satisfied through frequency hopping.

The wireless exciter detects and decodes command and control data usingthe data demodulator and decoder module (7-70) and the decode commandand control messages module (7-74). The command and control data is usedby the wireless exciter to configure the RF synthesizer (7-62), themodulator (7-31), the self calibration and ACL control loop (7-58), andtransmit power manual setting subsystem (7-86). The data encoder andmodulator (7-31) detects and re-modulates transmit data per a standard,followed by up conversion (7-34) and the automatic gain control loop(7-42, 7-60, 7-44,7-48) that is managed through the ALC loop controlmodule (7-58). The amplified signal (7-52) is followed by the transmitantenna feed and patch (7-54). The exciter additionally has interfacesto at least one external alarm (7-90), which are interfaced to theinternal processor (7-80).

A hybrid wireless/wireline exciter in accordance with an embodiment ofthe invention is illustrated in FIG. 19. In addition to the featuressupported by the previously described wired and/or wireless exciters,this design is able to wirelessly return status (19-9) or sensor (19-12)information via the same waveform nominally used by an RFID tag. Thehybrid exciter consists of a wire interface (nominally coaxial cable)(19-1), a receive antenna interface (19-2), a frequency conversion block(19-5), and a wire output (daisy chain) interface (19-6). The wirelessor wired interface signal undergoes frequency down conversion using amixing frequency created by the receive frequency synthesizer (19-10).After analog to digital conversion (19-7) a digital processor (19-8)constructs modulated waveforms for digital to analog conversion (19-13).These waveforms may describe tag commands (i.e., signals used tomanipulate tags) or exciter status information (i.e. informationtransmitted on a return channel to an RFID receiver system). In manyembodiments, exciter status information includes sensor trigger eventdata (19-11,12), exciter calibration information, and/or any otherinformation describing the present state of the exciter or itssurrounding environment. Low pass filters (19-14, 19-15) precedefrequency up conversion (19-16) per a mix frequency determined bysynthesizer block (19-19). A variable gain block (19-17) calibrates theoutput power level to not exceed a given threshold (for instance 30 dBm)prior to bandpass filtering the output signal (19-18). The finalresulting signal is radiated through the transmit antenna (19-21).

The hybrid wireless/wireline exciter shown in FIG. 19 can generate awaveform similar to that of an illuminated tag to communicateinformation to an RFID receiver system. In many embodiments, the excitercan generate a waveform that emulates an illuminated RFID tag so thatthe same RFID receiver system hardware configuration can be used to bothdetect illuminated RFID tags and receive status signals from exciters.In other embodiments, the return channel from an exciter to an RFIDreceiver system is a conventional wireless communication channel and theRFID receiver utilizes separate receiver configuration/hardware forcommunicating with exciters and receiving information from RFID tags.

An antenna that can be used in the construction of a reader transmit andreceive array, or exciter transmit and receive element in accordancewith an embodiment of the invention is illustrated in FIG. 8. Theantenna is fabricated using a brass, copper or Aluminum plate (8-2). Theplate (8-2) has four slots (8-22, 8-24, 8-28, 8-30), which end with acircular cut through (8-4, 8-6, 8-8, 8-26). There are two through holes(8-10, 8-12) for connecting the transmit and receive feeds, and fourthrough holes (8-14, 8-16, 8-18, 8-20) for plastic standoffs.

An antenna element similar to the antenna shown in FIG. 8 mounted in ahousing in accordance with an embodiment of the invention is shown inFIG. 9. In the antenna assembly (9-2), bushings (9-10, 9-12) connect theantenna element (9-4) to the Printed Circuit Board (PCB) (9-18), whichalso serves as the antenna ground plane. The connection is through thebushing's pins (9-14, 9-16) to the PCB and through screws (9-6, 9-8) tothe element. The antenna element is covered using a radome cover (9-11)at a fixed distance from the element. Plastic pins (9-5, 9-9) providethe antenna element with additional stability.

A receive array configuration in accordance with an embodiment of theinvention is depicted in FIGS. 10 and 11. The array is composed of fourelements, similar to the antenna assemblies (9-2) shown in FIG. 9. Thefour antenna elements (10-4, 10-6, 10-8, 10-10) are connected to theantenna PCB (10-2). FIG. 11 shows the cross-sectional view of the PCB(10-2), bushings (11-8, 11-10,), elements (10-4, 10-6, 10-8, 10-10), andthe radome cover (11-2). In the illustrated embodiment, the elementcenter-to-center separation is 164 mm (10-5). In other embodiments, theelement center-to-center separation is determined according to therequirements of the application.

Although specific antenna configurations are shown in FIGS. 8-11, otherantenna configurations capable of transmitting and/or receiving signalsin accordance with a specific application can be used in embodiments ofthe invention.

In a number of embodiments, the operation of exciters in a distributedarchitecture is managed and controlled by the RFID system using command,control and processing algorithms. A series of processes that arecoordinated by the RFID system to control the operation of distributedexciters in accordance with an embodiment of the invention isillustrated in FIG. 14. The processes (14-1) include an exciter networkinterface and control process (14-2), which provides the controlmessages, and manages communication protocols. In many embodiments,various processes determine the manner in which the exciters are to becontrolled and the exciter network interface and control process (14-2)is used to communicate the control information to the exciters. In theillustrated embodiment, exciter transmit power control process (14-4),optimizes, controls, manages, and calibrates the transmit power fromeach exciter, as specified by the user. In several embodiments, messagescontaining transmit power control information are provided to theexciters using the exciter network interface and control process (14-2).

A reader to exciter frequency hopping and management process (14-6),combines with an exciter management, scheduling and optimization process(14-10), and RFID frequency reuse, planning and optimization process(14-8), to optimize the operations of single and multi-system RFIDreceiver system deployment. The exciter frequency hopping and managementprocess (14-6) coordinates frequency hopping. In several embodiments,the frequency hopping and management process assigns random frequenciesto active exciters. In other embodiments, the process operates inconjunction with the RFID frequency reuse, planning and optimizationprocess (14-8) to employ an algorithm that optimizes frequency reusebased upon exciter location. In many embodiments, other algorithmsappropriate to the application are employed for the assignment offrequencies. In several embodiments, the exciter management, schedulingand optimization process (14-10) coordinates the activation of exciters.In a number of embodiments, the process periodically polls exciters. Inseveral embodiments, sensors detect the likely presence of items bearingRFID tags within an interrogation zone of an exciter and the sensorinformation is used by the process in the controlled activation ofindividual exciters.

An RFID false read discriminator process (14-12), detects and flags RFIDtag's which don't belong to a specified interrogation space. An RFIDfalse read descriminator process in accordance with an embodiment of theinvention is illustrated in FIG. 15. The process (15-1) obtains (15-4)sensor data from the interrogation space, detects (15-06) RFID tag data,which includes a tag's identification code, and determines (15-8) RFfeatures of detected tag information, which include signal strength,signal to noise ratio (SNR) and direction of arrival. The process usesthe collected sensor data, RFID tag data and RF features to determinewhether the RFID tag data was read from a tag located outside of theinterrogation space of the exciter that was activated by the RFIDsystem. A variety of processes can be used to determine whether RFID tagdata was read from a tag located outside an interrogation space basedupon collected information similar to the information described above.Specific processes are discussed further below.

A deployed RFID system that includes a distributed exciter architecturein accordance with an embodiment of the invention is illustrated in FIG.16. The deployment includes three interrogation spaces (16-14, 16-16,16-18). In this deployment each interrogation space utilizes twoexciters (16-2, 16-4 & 16-6, 16-8, & 16-10, 16-12). In many embodiments,the RFID system illuminates each interrogation space utilizing processessimilar to the process illustrated in FIG. 15 to identify whether RFIDtag data was read from a tag located within an exciter's interrogationspace.

For example, when intending to read an RFID tag in a first interrogationspace (16-16) the reader (16-22) can read tag “x” (16-34), and tag “y”(16-36). Data can be collected from other exciters, (e.g. tag “x” wasread by exciters (16-6 & 16-8), while tag “y”, was read by exciter(16-10, 16-6 & 16-12)), and the SNR for each signal compared (e.g., theSNR for tag “y” was lower compared to tag “x” when using exciters (16-6& 16-8)). Using the collected information, an RFID application servercan conclude that tag “y” (16-36), does belong to the firstinterrogation space (16-16).

The process used to determine whether RFID tag data is associated withan RFID tag located within an interrogation space can depend upon theapplication. In several embodiments, read rate information is used toidentify relationships between RFID tags and exciters. Various processesthat rely on read rates to draw conclusions concerning the location ofRFID tags in accordance with embodiments of the invention are discussedbelow.

Many processes in accordance with embodiments of the invention determinethe location of RFID tags from which information has been received bygathering information regarding RFID tag read rates and combine the readrate information with a topologic description of exciters and regions inorder to determine tag location. Combining read rates with a topologicdescription can enable false read detection when a tag is not locatedwithin a region of interest by approaching the problem of readdiscrimination in terms of ‘event sensing’. In particular the RFIDsystem is interested in events that involve tags moving from one‘hypothesis region’ to another. These events can be called ‘transitionevents’. In several embodiments, probabilities of transition events (ortransition hypotheses) inform the read discrimination process.

A transition hypothesis can be determined by defining the quantityp(x_(a)|y_({poll,sense}) ^(e) ^(j) ) as the posteriori probability thata tag is in hypothesis region x_(a) using tag observationsy_({poll,sense}) ^(e) ^(j) taken due to exciter e_(j), which is ineither a polling (poll) or a sensor (sense) driven mode. An exciter in apolling driven mode is activated by an RFID receiver system in a waythat is strictly periodic. An exciter in sensor driven mode is enabledwhen a sensor event (such as a light beam break or machine visionmotion) is detected. The majority of the time, exciters run in a pollingmode in which access to the RFID receiver system is divided in timeequally between the set of possible exciters. This occurs until asensing event is triggered, at which point a subset of exciters isgranted exclusive access to the RIFD receiver system.

An event associated with a transition hypothesis can be illustratedpictorially. A series of regions of interest and a plurality ofdistributed exciters are shown in FIG. 20. In the illustratedembodiment, an item bearing an RFID tag (20-1) moves from a firstlocation x₁ to a second location x₅. The posteriori probability that atag has moved from hypothesis region x₁ or x₃ to region x₅ can bedetermined by evaluating the probability:

$\frac{\begin{matrix}{\left( {{p\left( y_{poll}^{e_{1}} \middle| x_{1} \right)} + {p\left( y_{sense}^{e_{1}} \middle| x_{5} \right)}} \right)\left( {{p\left( y_{poll}^{e_{2}} \middle| x_{1} \right)} + {p\left( y_{sense}^{e_{2}} \middle| x_{5} \right)}} \right)} \\{\left( {{p\left( y_{poll}^{e_{1}} \middle| x_{3} \right)} + {p\left( y_{sense}^{e_{1}} \middle| x_{5} \right)}} \right)\left( {{p\left( y_{poll}^{e_{2}} \middle| x_{3} \right)} + {p\left( y_{sense}^{e_{2}} \middle| x_{5} \right)}} \right)}\end{matrix}}{C_{x}{p(y)}}$

The normalizing parameters C_(x) and p(y) can be dropped (normalizationcan be handled as a final separate step). The probability of similartransition events can be described more generally with the followingproduct of sums:

$\prod\limits_{a \in A_{origins}}{\prod\limits_{e \in E_{dest}}\left( {{p\left( y_{poll}^{e} \middle| x_{a} \right)} + {p\left( y_{sense}^{e} \middle| x_{dest} \right)}} \right)}$

A_(origin) ≡Set of hypotheses that can transition to the destinationhypothesis

E_(dest)≡Set of exciters surrounding the destination hypothesis

The remainder of this description focuses on the method through whichthe system obtains sums of the form p(y_(poll) ^(e)|x_(a))+p(y_(sense)^(e)|x_(dest)).

We now refer to FIG. 21 which provides a topological description ofexciter e^(j) in relation to hypothesis region x_(i). For each suchpairing, it is possible to determine the typical excitation link margin(the amount of power arriving at a tag above and beyond the absoluteminimum power required to activate the same tag). This link margin iswell approximated with knowledge of excitation power level, P_(t), anglefrom exciter boresight θ, mean distance from exciter to hypothesis, d,and the expected exciter and tag radiation patterns.

Referring now to FIG. 22, the resulting excitation link margin from FIG.21 is used to generate a probability mass function (pmf) that describesthe likelihood that a tag will be read a given percentage of the time(Read Rate) if it is located within hypothesis region x_(i). Read Rate(RR) is determined empirically by counting the number of times a tag isread in a fixed time interval and dividing this quantity by the numberof total possible reads that were possible in the same time duration(note that a read rate is labeled according to the triplet of exciterID, hypothesis region, and tag ID). Read Rates will be indexed byexciter (e) and hypothesis region (x_(j)) using notation RR_(e,x) _(j) .The related Read Rates are taken by an exciter (e) running in polling orsensor driven modes implicitly as determined by the location of ahypothesis region. Destination hypothesis regions are typically readusing a sensor driven mode. Given the preceding definitions it ispossible to specify the products of interest as a point on a Gaussianprobability mass function,

${{p\left( y_{poll}^{e} \middle| x_{a} \right)} + {p\left( y_{sense}^{e} \middle| x_{dest} \right)}} = {{\frac{1}{\sqrt{2\pi \; \sigma_{e,x_{a}}^{2}}}e^{\frac{- {({{RR}_{e,x_{a}} - \mu_{e,x_{a}}})}^{2}}{2\sigma_{e,x_{a}}^{2}}}} + {\frac{1}{\sqrt{2{\pi\sigma}_{e,x_{dest}}^{2}}}e^{\frac{- {({{RR}_{e,x_{dest}} - \mu_{e,x_{dest}}})}^{2}}{2\sigma_{e,x_{dest}}^{2}}}}}$

which is simply the product of two Gaussians multiplied together. Notethat prior to performing transition probability product of sums, allprobabilities associated with a given exciter, e, are normalized suchthat:

${\sum\limits_{a \in H}\left( {{p\left( y_{poll}^{e} \middle| x_{a} \right)} + {p\left( y_{sense}^{e} \middle| x_{dest} \right)}} \right)} = 1$

-   -   H≡Set of all hypothesis regions

The process described above can also include a model for the probabilityof reading a tag at a given location relative to the exciter's beam andtag environment (presence of absorbing material, etc). Such a modelsubsumes statistics on the spatial multipath field, whose large scalestructure is somewhat sampled by frequency hopping. This predictedprobability can be used at each read opportunity to update the Bayesianestimate for each hypothesis, whether the tag was read or not. Each ofthese hypotheses has a particular spatial trajectory versus time; sometags are consider static (at the same location for all measurements),and some are moving (usually at constant velocity in a particulardirection, such as through a door or loading area). Since sensorsexternal to the RFID system are used to align the conjectured movingtrajectories in time, a simplifying approximation can be made to theevery-read-opportunity approach, namely that read fraction statisticscan be kept during key time intervals surrounding the events. Thestatistics on these read fraction averages often follow Poissonstatistics, based on the individual probability of a read and the numberof opportunities to read in the interval. (An exception to this is thecase of static tags, where there is a high correlation between readfraction over time; this correlation can be taken into account with aspatial correlation function, which has correlation distance of roughlya wavelength/half-wavelength). In our preferred embodiment (describedabove), we approximate Poisson statistics with a Gaussian distributionon read fraction.

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof.Accordingly, the scope of the invention should be determined not by theembodiments illustrated, but by the appended claims and theirequivalents.

What is claimed:
 1. An RFID system configured to manipulate RFID tags,comprising: an RFID receiver system configured to read RFID taginformation from activated RFID tags within a receive coverage area; anda plurality of exciters defining a plurality of interrogation spaceswithin the receive coverage area of the RFID receiver system, where atleast one of the plurality of exciters is able to activate an RFID tagwithin the plurality of interrogation spaces and the plurality ofinterrogation spaces are contained within the receive coverage area ofthe RFID receiver system; wherein the RFID receiver system is configuredto transmit a control signal that identifies one of the plurality ofexciters and includes information indicative of an RFID taginterrogation signal; wherein the plurality of exciters are configuredto receive the control signal; wherein the exciter identified in thecontrol signal is configured to illuminate an interrogation space withthe RFID tag interrogation signal and activate an RFID tag within theinterrogation space; and wherein the RFID receiver system is configuredto read RFID tag information from an RFID tag activated by an RFID taginterrogation signal generated by one of the plurality of exciters.